The present invention relates to cable television, and more particularly, to cable television signal transmission.
Cable television systems generally require a conversion system for frequency converting the transmitted channels from baseband frequencies to their designated RF frequencies for transmission over the cable medium. This system is typically part of a cable television (CATV) headend system, where the composite, multi-channel CATV signal is generated and amplified for distribution to customers.
Within existing headend systems, each frequency converters typically use multiple analog mixing stages, with one or more analog phase-locked loops (PLL) to generate the local oscillators. One converter is required for each channel, and there can be more than one hundred channels in a typical CATV system. These systems are often costly and require significant amounts of hardware. Additionally within existing systems, control of signal amplitude for each channel can be complex.
What is needed is a cost-effective system and method for frequency converting baseband television signals and creating composite, multi-channel CATV signals within a CATV headend system.
The invention is directed to systems and methods for digital upconversion of baseband television signals and other types of signals, such as those associated with cable modems, that can be used in cable television headend systems. In one embodiment, the digital headend upconversion system includes a demultiplexer, a digital frequency adjustment system and a digital to analog (DAC) conversion system. In one embodiment the digital frequency adjustment system includes a set of upsample and upconversion elements that shift upwards the frequency of baseband signals. In another embodiment, a tree structure of sets of upsample and upconversion elements is used. The digital to analog conversion system includes a single digital to analog converter or a set of converters.
An alternative embodiment of the digital headend upconversion system is a digital hybrid headend upconversion system that includes a demultiplexer, a digital frequency adjustment system and an analog frequency adjustment system. In this embodiment, the frequencies of baseband signals that are input to the upconversion system are partially upshifted within the digital domain and partially upshifted within the analog domain. The digital frequency adjustment system is as described above, except that the frequencies of the baseband signals are partially adjusted rather than upshifted to final desired frequencies for distribution. The analog frequency adjustment system includes a set of digital to analog converters followed by a set of band pass filters, followed by a set of mixers, followed by another set of band pass filters, followed by another set of mixers, and finally followed by a set of low pass filters. The outputs of each of the low pass filters are summed together to form the desired frequency upconverted composite signal for distribution throughout a cable network. In a further feature, within a digital or digital hybrid upconversion system, an individual channel gain adjustment system can be included to allow precise gain adjustment controls for individual channels.
Methods for digital upconversion of television signals are also provided. In one embodiment, the method includes receiving digital baseband television signals, demuxing those signals, upsampling and upconverting the demuxed signals, then recombining the signals and performing a digital to analog conversion. In one embodiment, upsampling and upconverting the demuxed signals occurs in a two steps. In alternative embodiments, a tree structure of upsampling and upconversion elements in used, such that upsampling and upconverting occurs in multiple two-step phases.
In another embodiment, the frequencies of digital input signals are partially upshifted within the digital domain and partially upshifted within the analog domain.
Use of the invention provides two principal benefits. First, use of the invention reduces the cost and complexity of hardware needed for a cable television headend system. Second, use of the invention simplifies digital control of channel amplitude for the television signals.
Further embodiments, features, and advantages of the invention, as well as the structure and operation of the various embodiments of the invention are described in detail below with reference to accompanying drawings.